For each sub-case, we need to check 4 conditions:

**Condition 1:** Every color appears in a contiguous range of $x$'s.

We can check this condition by maintaining an array that describes at which $x$ a color last appeared. If the last $x$ is always 1 less than the current $x$, then this condition holds.

Sub-case 2 in the example does not satisfy this condition.

**Condition 2:** For each $x$, if one appearance of color $a$ is after the
first appearance and before the second appearance of color $b$, then the other
appearance of color $a$ must do the same.

Let's say that a color becomes "activated" when it appears for the first time, and then "deactivated" when it appears the second time. We can then check this condition by maintaining a stack of active colors. When a color becomes deactivated, it must be at the top of the stack. If this is always true, then this condition holds.

Sub-case 3 in the example does not satisfy this condition.

If the 2 appearances of color $a$ are between the 2 appearances of color $b$, then we know that bracelet $a$ must be contained inside bracelet $b$. This hierarchical relationship allows us to transform the bracelets into a tree structure, where:

- The nodes are the bracelets.
- If a bracelet is contained by another bracelet, then its parent is the innermost bracelet that contains it.
- Otherwise, its parent is a placeholder node (e.g. node $0$).

**Condition 3:** Each node's parent in the tree must be consistent for every
$x$ it appears in.

This is because if some color appears, then its parent must also appear. We can check this condition by maintaining an array describing the parents of colors we've seen before. We can then check this array against the input for each $x$.

**Condition 4:** The ordering of colors in the input must be consistent for
every $x$ it appears in.

We can check this condition by maintaining 2 lists – one for the current $x$'s color order and the other for all previous $x$'s' color order. To check the ordering, we can compare the ordering of these two lists' intersection, and then merge the lists after processing $x$ by using 2 pointers. Since the constraints are so small, straightforward list insertion is fast enough for this.

Sub-case 5 in the example does not satisfy this condition.

Andi's code (which runs in $\mathcal{O}(N^2M)$ time).

#include <algorithm> #include <iostream> #include <stack> #include <vector> using namespace std; int main() { cin.tie(0)->sync_with_stdio(0); int t, last_appeared[51], parent[51]; cin >> t; while (t--) { int n, m; bool good = true; vector<int> glob_order; cin >> n >> m; for (int i = 0; i <= n; i++) last_appeared[i] = parent[i] = -1; while (m--) { int k, curr = 0; cin >> k; stack<int> stck; vector<int> curr_order; while (k--) { int c; cin >> c; if (!good) continue; if (last_appeared[c] == m && stck.top() == c) { stck.pop(); curr = parent[curr]; } else { if ((~last_appeared[c] && last_appeared[c] != m + 1) || last_appeared[c] == m || (~last_appeared[c] && parent[c] != curr)) { // Conditions 1, 2, and 3 good = false; continue; } // Condition 4 part A curr_order.push_back(c); parent[c] = curr; last_appeared[c] = m; stck.push(c); curr = c; } } // Condition 4 part B if (!good) continue; int ptr = 0; for (int i : curr_order) { while (ptr != glob_order.size() && !count(curr_order.begin(), curr_order.end(), glob_order[ptr])) ptr++; if (!count(glob_order.begin(), glob_order.end(), i)) glob_order.insert(glob_order.begin() + ptr, i); else if (ptr == glob_order.size() || glob_order[ptr] != i) { good = false; break; } ptr++; } } cout << (good ? "YES\n" : "NO\n"); } return 0; }

Of course, it is possible to solve this problem in $\mathcal{O}(NM)$ time, but it was not necessary to do so due to the low constraints.